Hydraulic fan drive systems are well known in the art to drive cooling fans. In one such arrangement two different fluid motors are used in combination to provide the needed torque at higher speed. At lower speeds only one of the fluid motors is used. In other arrangements, activation of and the speed of the cooling fan is controlled in response to the temperature of the working fluid in the system. In many fan drive systems, a variable displacement pump is used to drive the fluid motor and the speed of the fluid motor is controlled by varying the displacement of the variable displacement pump. In these systems, the pump control signal is directed through a variable orifice and then subsequently through a fixed orifice to the tank or through a fixed orifice first and then through a variable orifice. The variable orifice may be controlled by a spool valve that is movable in response to a remote signal, either manual or electrical. The metering of the control signal to tank produces undesirable heat and also many times requires larger pumps since there is a constant amount of flow being passed to the tank. Additionally, using small orifices to reduce the flow loss results in an orifice that is sensitive to plugging from debris in the fluid. Likewise, when using this type of control, it may also be necessary to provide a closed logic for the fan speed. Typically, U.S. Pat. No. 5,876,185 issued Mar. 2, 1999 to Schimpf et al. teaches an arrangement that modulates the control pressure to the pump control for control of pump displacement in a given direction and bleeds off fluid flow only to change pump displacement in the opposite direction. A variable force is provided to the control spool in opposition to the spring control force in order to provide the modulated control pressure to the pump control. Additionally, the discharge pressure of the pump is applied against a differential area of the control spool in order to provide a force against the spring control force that is directly related to the discharge pressure. The control pressure to the pump control generally decreases as the discharge pressure increases but is not linear. This can cause pump control instability that requires spool dampening orifices to be added. Otherwise the control instability reduces the fan drive component life. Since the maximum pump pressure is controlled solely by the one spool and if dampening is added then the maximum pressure can overshoot again resulting in pump and motor component life reduction as well as possible life reduction in valves and components.
Accordingly, the present invention is directed to overcoming one or more of the problems as set forth above.